The present invention relates to a short circuiting transfer arc welding machine, and more particularly to a short circuiting transfer arc welding machine capable of supplying a proper current dependent on the load condition between a wire electrode (hereinafter referred to as a "wire" and) a workpiece.
FIG. 1 of the accompanying drawings illustrates a conventional short circuiting arc welding machine. The short circuiting arc welding machine is composed of a DC power supply circuit 1 supplied with commercial three-phase AC power, a switching element 2 such as a transistor, a DC reactor 3, a diode 4 for limiting a surge voltage produced by the DC reactor 3 immediately after the switching element 2 is cut off, an auxiliary power supply 5 for supplying a direct current to maintain an arc, a wire supply reel 6 for supplying a wire 7, a motot 8 for feeding the wire 7, and a welding torch 9 for welding a workpiece 10. The arc welding machine also includes a switching control circuit 12 for generating command signals for opening and closing the switching element 2, a voltage detector 15 for detecting a voltage V between the wire 7 and the workpiece 10, a first decision circuit 16 for comparing the voltage V as detected by the voltage detector 15 and a reference voltage Vo and for issuing a signal for increasing a welding current to the switching control circuit 12 when V.ltoreq.Vo, and a second decision circuit 17 for comparing the voltage V as detected by the voltage detector 15 and a reference voltage Va and for energizing a timer 18 and issuing, upon elapse of an interval of time T, a signal to reduce the welding signal to the switching control circuit 12 when V.gtoreq.Va.
FIG. 2 (a) is a diagram showing the waveform of the voltage applied between the wire 7 and the workpiece 10, FIG. 2(b) is a diagram showing the waveform of the welding current, FIG. 2(c) is a view showing an arc condition, and FIG. 2(d) is a cross-sectional view of a weld bead.
The operation of the prior art welding machine will now be described. First, the DC power supply circuit 1 and the auxiliary power supply 5 are energized and the motor 8 is driven for feeding the wire 7 until its end and the workpiece 10 form a short circuit. The voltage V as detected by the voltage detector 15 is now smaller than the reference voltage Vo, and the first decision circuit 16 issues a signal to the switching control circuit 12 to increase the welding current. The switch control circuit 12 then closes the switching element 2. As shown in FIGS. 2(a) and 2(b), the welding current increases during a period A until the short circuit between the wire 7 and the workpiece 10 is eliminated and an arc is produced therebetween. When the arc is generated, the voltage detected by the voltage detector 15 is equal to an arc voltage. The output from the first decision circuit 16 is then cut off, and the second decision circuit 17 produces a signal to energize the timer 18. Upon elapse of the time delay T after the voltage V from the voltage detector 15 has exceeded the reference voltage Va, the switching control circuit 12 is responsive to a signal from the timer 18 to turn off the switching element 2. The welding current is now reduced through the DC reactor 3, the diode 4, the workpiece 10, and the wire 7, until finally current flows only from the auxiliary power supply 5. The foregoing cycle of short circuiting and arcing is repeated to melt the wire 7 and the workpiece 10 and transfer the molten material from the wire 7 to the workpiece 10, thereby forming a weld.
As shown in FIG. 2(b), during an arcing period B, the welding current varies relatively smoothly and the arc flame tends to spread as shown in FIG. 2(c). This results in a reduced arc current density on the surface of the workpiece, and a weld bead with small penetration into the workpiece as shown in FIG. 2(d). This problem manifests itself where the polarity (known as "straight polarity") of the arc is such that the wire 7 is rendered negative and the arc current density on the workpiece surface is lower. The weld cannot be sufficiently strong, and the speed of the welding operation cannot be increased due to a reduced amount of bead penetration into the workpiece.